Detecting hidden explosive devices in war zones and transportation hubs is an important pursuit. Commonly-used, highly energetic compounds found in explosive formulations include 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitrotriazinane (RDX), and pentaerythritol tetranitrate (PETN). Existing technologies for detecting the energetic chemical components of explosive devices, including analytical spot tests, fluorescent sensors using either small-molecule fluorophores or fluorescent conjugated polymers, chemiresistive sensors, portable mass spectrometers, and X-ray systems, often have limitations. For instance, while X-ray systems are capable of detecting bulk hidden explosive devices and portable mass spectrometers are capable of identifying the exact chemical structures of suspect chemicals, the practical deployment and/or longevity of these hardware-intensive technologies in complex environments is non-trivial.
Fluorescent sensors are comparatively technology-unintensive, have desirably low detection limits and the ability to identify (e.g., respond to) entire classes of molecules (such as nitroaromatics) or particular functional groups (vide infra). Chemical spot tests can be specific but are not as sensitive as fluorescent sensors and generally do not have the analytical advantages of an emissive signal, such as remote line-of-sight (e.g., stand-off) detection or prospects for complex signal processing (i.e., fluorescence lifetimes, depolarization).
Nitroesters and nitramines have been known to degrade under highly acidic or basic conditions, and methods for detect these chemical degradation products have been studied. The base-promoted digestion of nitroglycerin (NG) has also been studied and is thought to evolve a mixture of nitrate and nitrite anions, among other degradation products. (FIG. 1A) Similarly, RDX is also known to decompose in basic media and produce nitrite ions. The commercially-available Greiss test for nitrite ions has been employed to detect the evolution of nitrite upon base-promoted degradation of RDX and PETN. (FIG. 1B) FIG. 1C shows the components of the Greiss test, including sulfanilamide 4 and arylamine 5. The Greiss test involves the reaction of sulfanilamide 4 with nitrite to form diazonium salt 6, which then reacts with an arylamine 5 to form a brightly-colored azo dye (7). Similar tests conducted in the absence of a base have indicated that nitrite ions may be generated upon the photolysis of RDX and PETN.
Unfortunately, the Greiss test or variations thereof have certain disadvantages in the detection of explosives such as RDX and PETN. For example, simple standoff detection (i.e., detection at a distance) with colorimetric spot tests is limited by the difficulty in getting a clear optical signal returned from a purely absorptive process. Moreover, even with optimized reagent systems, the detection limit of the Greiss test is in the microgram regime, which is generally not competitive with existing methods to detect RDX and PETN.